1. Field of the Invention
This invention relates to a thermal sensing system and more particularly to both imaging and non-imaging sensing systems incorporating an array of photon-detecting elements.
2. Discussion of Prior Art
Thermal imaging systems are known in the prior art. Such imaging systems can involve either series or parallel processing. In the former case a scene is scanned and each component of the scene is focused sequentially onto a detector. These systems are not easy to design however if compactness is important: the scanning mechanism renders the adaptation to lightweight imagers extremely difficult. An alternative arrangement for area imaging is to employ many detectors to sample simultaneously distinct sections of the scene. A major disadvantage of this system is that the transfer function from incident infrared flux to output signal (detector signal) is particularly sensitive to variation between detecting elements. This results in an image degraded by fixed pattern noise arising from sources both within and independent of the detecting elements. Imperfections in the optical system (e.g. vignetting) and variations in the associated electronic circuits are examples of the latter case. Photodetector sources can be static variations in characteristics (e.g. area, quantum efficiency or cut-off wavelength) or dynamic instabilities (temperature, offset voltage and slope resistance all drift over a period of time) which give rise to the need for regular array recalibration. Additionally l/f noise introduces an error which increases with the period between calibrations. Compensation for inter-detector variations is particularly important in "staring" applications which measure the absolute radiation intensity within a scene. Scanning imagers measure only changes in intensity across a scene. The output from a staring array is thus of poor contrast in comparison.
Non-imaging thermal detectors are also known in the prior art. They have applications in areas such as robotics and missile guidance systems for which human interpretation of detector output is not required. The actual detecting elements are similar to those described above in relation to imaging systems. In non-imaging systems however an object (robot or missile) is arranged to respond to a particular signal appearing on the detectors. This recognition feature may vary in its complexity. For example, pattern recognition can be linked to a number of response options or a less complex reflex can result in steering towards the achievement of a characteristic detector response. Staring arrays are particularly suitable in satisfying the lightweight requirements of missile systems. However in such missiles the detector system is subject to rapid temperature change as the missile cone heats up during flight. Frequent recalibration is necessary in order to maintain an acceptable accuracy.
An imaging system incorporating a detector array is disclosed by P. N. J. Dennis et al. in Proc. SPIE 572 22 (1985). The authors describe a two dimensional close packed array of cadmium mercury telluride detectors interfaced to a silicon charge coupled device (CCD). Infrared light incident on a detector elicits a response signal which is injected into the CCD and integrated over a period of time (the stare time). The subsequent signal processing system addresses the fundamental problems of poor contrast from the infrared scene and nonuniformity of detector element responses. The nonuniformity correction is made by exposing the array to two uniform scenes of different temperature with an arrangement of mirrors used to introduce them into the optical path. From measurements of stimulus infrared flux and detector response a correction factor is derived for each individual detector by forcing a uniform scene to give rise to a uniform image. The signal response is fitted linearly to incident radiation intensity and an offset and gradient derived to describe the transfer function for each detector in the array. All values of signal response at all detectors can thus be converted into corrected incident flux values. Array calibration in this way is performed periodically (perhaps hourly or daily) and the updated correction factors applied to subsequent measurements. This compensates for l/f noise and detector parameters drifting over a period of time as a result of, for example, temperature changes.
A disadvantage of such sensing systems is that possible reference temperature sources limit the performance in terms of speed and compactness. If physically separate reference scenes are used then the sensor requires an optical system with considerable complexity and bulk. Alternatively reference temperatures could be supplied by a Peltier cooled/heated reference plane but the finite time taken to adjust to temperature leads to a lengthy calibration process.
A combination of both these techniques is disclosed in U.S. Pat. No. 4,419,692. This patent is concerned with a multi-detector scanning thermal imager which already possesses a bulky scanning mechanism and so no particular advantage is gained by physical reduction of the reference system. Scanning thermal imagers are frequently used with an array of detector elements in order to increase sensitivity. Uniformity corrections then have to be incorporated into the signal processing. In this device errors are reduced over the thermal range present in the scene under observation by allowing one of three thermo-electric references to be varied by the operator to provide a reference level at the midpoint of the scene thermal range. The Peltier cooler used to provide this third reference is varied between scenes in order to adapt the calibration technique to the characteristics of each particular scene. Three reference temperatures are used as opposed to two in the Dennis system above: one provides a dc level about which the ac temperature variations detected by the scanning imager are referenced and the remaining two provide the uniformity correction for the detector array.
Another correction mechanism, again used with a scanning thermal imager, is described in UK patent 2 225 914 A. A single reference source is used which removes the requirement for additional optical components to bring different sources into the optical path. A disc coated to provide regions of differing reflectivity is rotated between this single reference source and the thermal imager. The differing reflectivities permit different proportions of the radiation emitted from the source to be passed to the imager. Thus different effective temperatures can be used as references. The mechanical spinning of the disc is synchronised with the scanning mechanism to arrange for the reference temperatures to be passed to the thermal imager only during inactive periods of the scan. However this technique still requires mechanical movement mechanisms for both the disc spinning and scene scanning. Overall, this imager is still bulky and unsuitable for some applications. Furthermore there is a limit to the number of differing-reflectivity sections which can be incorporated on the disc. Each level of flux must be viewed for sufficient time to allow the imager to adjust to the new reading and the majority of the disc must be uncoated to correspond to the time needed to view the scene.